Mammalian knees wear out for a variety of reasons, including inflammation from arthritis, injury, or simple wear and tear. Over the past 40 years, total knee arthroplasties (commonly referred to as total knee replacements or “TKR”) have become the standard of care for end-stage arthritis. In most TKR procedures, the natural bearing surfaces of the upper portion of the tibia (tibial plateau) and the lower portion of the femur (femoral condyles) are resected and replaced with artificial material. Specifically, approximately between 0.5 to 1.5 centimeters of the upper portion of the tibia, including both the intercondylar eminence and the medial and lateral tibial plateaus, are resected, leaving a relatively flat surface onto which a rigid support member is affixed. Then, a synthetic surface portion is affixed to the support member, with the surface portion simulating the intercondylar eminence and the tibial plateaus. A lower end portion of the femur is then resected, and a member having bearing surfaces replicating the femoral condyles is then affixed to the remaining end of the femur.
The majority of currently available prosthetic knee implants employed for TKR do not retain the natural anterior cruciate ligament. Rather, during the implant procedure, the anterior cruciate ligament is either removed or, if preservation is attempted, has been found to rupture shortly after implant of the prosthesis. This is particularly true for TKR candidates, who often have a sacrificed anterior cruciate ligament going into the surgery. As a result, the mechanical interaction between the femoral and tibial components in a TKR is the primary means to stabilize the anterior-posterior motion of the knee.
Although the existing TKR prostheses succeed in increasing patient mobility, and provide the patient with the desired therapeutic result, at least one significant disadvantage remains. Namely, in a TKR wherein the anterior cruciate ligament is lacking, the femoral condyles translate in a posterior direction in full extension and translate in an anterior direction in flexion, which is reverse of the motions in a natural knee joint. Such abnormal translation and pivot shift often results in the patient's compromised functional abilities, such as quadriceps avoidance, and changed upper body mechanics during activities such as stair climbing and rising from a chair. Even asymptomatic patients show gait abnormalities that could lead to reduced functional ability to perform activities of daily living over time.
Further, abnormal anterior translation of the TKR can lead to accelerated wear of the prosthesis. Indeed, current TKR prostheses have a functional lifespan of approximately 15 years, such that younger patients (who are increasingly receiving TKRs) are more likely to require revision surgery as they age. The amount of bone loss that is inherent in a TKR makes a revision procedure much more difficult in the future as even more bone must be removed.
Existing TKR prostheses attempt to compensate for the loss of the anterior cruciate ligament by containing or limiting the amount of abnormal translation in the nonstabilized knee. For example, U.S. Pat. No. 5,413,604 discloses a TKR prosthesis wherein the anterior cruciate ligament must be sacrificed, thereby resulting in a nonstabilized TKR with abnormal anterior translation. Without an anterior cruciate ligament, the prosthesis relies primarily on the mechanical interaction between the femoral and tibial components as a means to accommodate the abnormal anterior translation and stabilize the knee. Similarly, U.S. Pat. No. 7,014,660 discloses a TKR prosthesis that incorporates a control arm and stop pin assembly to limit the amount of anterior sliding movement caused by the lack of the anterior cruciate ligament. Unfortunately, neither prosthesis disclosed in the '604 and '660 patents actually prevents the abnormal anterior translation of the nonstabilized TKR; or as shown in recent medical research pertaining to medial and lateral translation of ACL deficient knees, rather, they only attempt to accommodate or limit it. As a result, the patient's functional abilities remain limited, and the prosthesis is subject to premature wear and tear. The U.S. Pat. Nos. 5,413,604 and 7,014,660 are hereby incorporated by reference in their entireties.
Therefore, there exists a constant need in this art for an improved TKR prosthesis that allows for the replacement of an anterior cruciate ligament, and approaches the mobility, stability and longevity of a natural, healthy knee joint.